Japanese Patent Application Publication No. 2012-82747 discloses a piston pump/motor including a cylinder block that is fixed to a rotary shaft and includes a plurality of cylinder bores, a piston disposed to be free to slide in each cylinder bore such that a volume chamber is formed thereby, a swash plate that causes the piston to reciprocate as the cylinder block rotates such that the volume chamber expands and contracts, and a valve plate that slides against the cylinder block and includes an intake port and a discharge port communicating with the volume chamber.